The present invention relates to an apparatus and method to detect in an electrical cable a fault condition caused by a current surge.
In a typical electrical power distribution system, a main power cable may run underground the better part of a mile and feed as many as ten step-down transformers. If a fault occurs in the line or in one of the transformers, there is commonly current surge which operates a fuse at the origin of the line to cut off current through the line. The service problem is to find the location of the fault and repair it quickly and inexpensively so that the line may be returned to service.
It has been proposed in the prior art to have fault detectors at various locations in a power distribution system. Thus, the service crew will be able to check the various fault detectors to determine the location of the fault. However, to the best knowledge of the applicants herein, these prior art fault detecting devices have been unreliable. More often than is acceptable, they have given no fault indication when they should have, or have given a false indication when a fault current had not passed by them.
In addition to reliability there are other factors to be considered in designing a fault detecting system. For example, it would be desirable that it be entirely passive to that it would derive all of its operating power from the current in the line that it is monitoring. One way of accomplishing this is to have the recognizer in the detecting equipment respond to the fields associated with the current in the line. However, with a fault recognizer that flags all currents above a fixed threshold, there could be a false indication where there is simply inductive kick-back, or no detection for a fault which resulted from a marginal over current which existed for a relatively long duration. Also, it has been proposed in the prior art to provide a fault detector which has the fault detecting signal charge a capacitor, which in turn discharges to show a fault condition. This too has proven unreliable in that if the device is set to detect faults of a certain current magnitude, it will not reliably detect faults which result from current surges of a substantially different magnitude.
A search of the patent literature has disclosed a number of United States Patents. These are as follows.
U.S. Pat. No. 1,830,541, Harris, and U.S. Pat. No. 2,815,493, Kain, both disclose current transformers to measure current.
U.S. Pat. No. 3,777,217, Groce et al., discloses a system in which a current transformer generates a current to charge a first capacitor which in turn discharges to indicate normal current. A second capacitor is charged and fired to show when a current reaches a higher level. Another device which operates on generally this same principle is indicated in U.S. Pat. No. 3,512,045, Tipton et al.
U.S. Pat. No. 3,991,366, Schweitzer, Jr., shows a fault detector that employs a reed switch that is closed when a certain level of current is reached.
In view of the foregoing, it is an object of the present invention to provide a fault detecting apparatus and method which is able to reliably recognize substantially all faults, both fast and slow, which is able to be attached to a power distribution system without negative impact on its operation, which is able to operate for years without maintenance, and which is relatively small, inexpensive and easy to install.